Increased gate control, reduced body effect, reduced junction capacitance and lower junction leakage current are among the attributes of insulated gate field effect transistors (IGFETs) including metal-oxide-semiconductor field effect transistors (MOSFETs) fabricated in semiconductor-on-insulator (SOI) substrates. These attributes of SOI IGFETs have generated interest in their use in low voltage, low power applications. However, problems unique to the fabrication of devices on SOI substrates complicate the design of circuits thereon and add to the costs of development and production.
Unlike IGFET devices constructed on bulk silicon substrates which have bodies in electrical contact with the substrate and thereby exchange charge carriers with the substrate, SOI IGFETs have floating bodies which store charge carriers on a permanent or near permanent basis. This characteristic of SOI IGFET devices results in their electrical properties exhibiting hysteresis effects, wherein the SOI device behaves electrically as a function of its state during the preceding several hundred milliseconds of operation. In addition, the floating body of the SOI IGFET has been linked to reliability problems, such as increased susceptibility to device latch-up and increased hot carrier degradation as compared to conventional IGFETs built in bulk silicon substrates.
An obstacle to the acceptance of SOI IGFETs is the increased cost of SOI substrate fabrication, which involves additional process steps, for example, high dose oxygen implantation with long duration anneals at high temperature in silicon-implanted-with-oxygen (SIMOX) substrates, or the bonding of two prepared wafers and the polishing down of the top semiconductor layer to the desired thickness, as in bond-and-etchback SOI (BESOI).
Accordingly, it is an object of the invention to provide a field effect transistor having a substantially isolated body which nevertheless remains joined to a substrate for exchange of charge carriers therewith.
Another object of the invention is to provide a field effect transistor having a substantially isolated body to facilitate rapid switching speed, while maintaining body contact to substrate to avoid the deleterious results of the floating body described above.
Still another object of the invention is to provide a field effect transistor having a very short effective channel length.
Another object of the invention is to provide a field effect transistor having reduced junction capacitance.
A further object of the invention is to provide a method of fabricating a field effect transistor which has large channel width but occupies a small area of the semiconductor substrate.
Another object of the invention is to provide a field effect transistor having a body which has sublithographic thickness for providing enhanced device current.
Still another object of the invention is to provide a field effect transistor (FET) in which the gate conductor is wrapped around the body of the FET and the FET is electrically isolated from other circuit elements on the substrate by large isolation regions.